(1) Field of the Invention
The present invention relates to a high pressure discharge lamp using a ceramic discharge tube and a method of producing the same.
(2) Related Art Statement
In the high pressure discharge lamp using a ceramic discharge tube, both end portions of the ceramic discharge tube are closed by inserting clogging members (usually called "ceramic plug") at the inside thereof, a through-hole is bored in the clogging member, and a metallic electric conductor is inserted in the through-hole. The metallic electric conductor is provided with a given electrode, and an ionizable luminescent material is sealingly filled in the inner space of the ceramic discharge tube. As such a high pressure discharge lamp, a high pressure sodium luminescent lamp and a metal halide lamp are known. Particularly, the metal halide lamp has an excellent color-display property. By the use of the ceramic as the material for the discharge tube, the discharge tube has been possible to use at high temperatures.
FIG. 1 is a sectional view for illustrating a preferred example of the structure of the end portion of such a ceramic discharge tube. A main body 11 of the ceramic discharge tube has a tubular shape or a bottle shape throttled at the both ends each having a cylindrical end portion 12. The main body 11 and the cylindrical end portions 12 are made of, for example, a sintered alumina body. The inner surface 11a of the main body 11 has a curved shape. Since the inner surface 12a of the end portion 12 is straight viewed in the axial direction of the main body, a corner 36 is formed between the main body 11 and the end portion 12. A clogging member 41 is inserted and held inside the end portion 12 and has a through-hole 41a formed in the clogging member 41 and extending in the axial direction of the clogging member 41. A slender electric conductor 5 is fixedly inserted in the through-hole 41a. In this example, the electric conductor 5 has a cylindrical shape, and fashioned so as to introduce an ionizable luminescent material in an inner space 13 of the main body 11 through an inner space 5a of the electric conductor 5. An outer end of the electric conductor 5 is provided with a sealing portion 5b which seals and holds a starting gas and the ionizable luminescent material after the sealing therein. The gases are sealed inside the discharge tube by the sealed portion 5b. An electrode shaft 7 is joined to the outer surface of the electric conductor 5.
In such a ceramic discharge tube, it is necessary to effect sealing between the clogging member 41 and the cylindrical end portion 12 and between the clogging member 41 and the electric conductor 5. For that purpose in a preferred example, the electric conductor 5 is inserted in the through-hole of a calcined body of the clogging member 41 which is then inserted in the cylindrical end portion 12 to prepare an assembled body, and the assembled body is sintered to an integral body. At that time, the sealing between the cylindrical end portion 12 and the clogging member 41 as well as the sealing between the clogging member 41 and the electric conductor 5 are effected by the integral sintering.
In the above sealing method, the clogging member 41 and the cylindrical end portion 12 are designed in such a fashion that the inner diameter of the cylindrical end portion 12 becomes smaller than the outer diameter of the clogging member 41, if the calcined body of the cylindrical end portion 12 not having therein the inserted calcined body of the clogging member 41 is fired. Therefore, the clogging member 41 is firmly and tightly compressed and held in the cylindrical end portion 12. The same applies to the clogging member 41 and the electric conductor 5. As the material of the electric conductor, molybdenum, tungsten, rhenium or their alloys are advantageous from the viewpoint of corrosion resistant property. As the material of the ceramic discharge tube, alumina ceramics are usually used. If an alumina ceramic is used as the material of the clogging member, a difference between thermal expansions of the clogging member and the electric conductor becomes large, so that usage of composite materials made of alumina ceramics and the above described metals or other cermets have been known.
However, the inventors made further studies on the above preparation method to find out the following problems. Namely, in the step of the above final firing, the calcined body of the cylindrical end portion 12 and the calcined body of the clogging member 41 are certainly respectively fired and shrunk in the lateral direction of FIG. 1 (the circumferential direction of the ceramic discharge tube). The clogging member 41 and the electric conductor 5 are firmly held and sealed in the ceramic discharge tube by the firing shrinkage. However, in the step of the final firing, the calcined body of the cylindrical end portion 12 and the calcined body of the clogging member 41 are simultaneously fired and shrunk towards the direction of the arrow E (the direction of the central axis of the ceramic discharge tube). As a result, large thermal stresses are formed and remain viewed in the direction E of the central axis of the ceramic discharge tube between the clogging member 41 and the cylindrical end portion 12 and between the clogging member 41 and the electric conductor 5.
Particularly, if the high pressure discharge lamp has a superior color-display property and a coldest temperature of 700.degree. C. or more and subjected to on-off lighting cycles, the influence of the above residual stress is enlarged by the heating cycles, so that the ceramic discharge lamp is likely destructed to leak the ionizable luminescent material therefrom.
In addition, in the sealing structure of the end portion as shown in FIG. 1, the sealing between the clogging member 41 and the electric conductor 5 is affected basically by the pressure therebetween, so that a more high reliability of the sealing is necessary, considering a multiple number of repetition of on-off lighting cycles and a difference of thermal expansion coefficients of the clogging member 41 and the electric conductor 5. For that purpose, development of a sealing structure having a high corrosion resistant property and a high reliability against metal halides are earnestly requested.